(a) Technical Field
The present disclosure relates, generally, to a surface pressure controlling device for a fuel cell stack. More particularly, it relates to a surface pressure controlling device for a fuel cell stack, in which an inflator capable of being expanded by pneumatic or hydraulic pressure is suitably mounted in an end plate so as to control the surface pressure required for the assembly of the fuel cell stack to be maintained above a predetermined level.
(b) Background Art
Typically, a fuel cell stack comprises a plurality of unit cells, in which a membrane electrode assembly including an electrolyte membrane and anode and cathode electrodes, a gas diffusion layer (GDL), a gasket, a separator including a flow field, and end plates for supporting and fixing them are sequentially stacked, the plurality of unit cells being coupled together while maintaining a predetermined surface pressure. For example, about 100 to 300 unit cells are suitably arranged between the end plates.
Since the surface pressure in the fuel cell stack is directly related to the ohmic loss due to the increase in contact resistance and the mass transfer resistance in the gas diffusion layer, it is necessary to maintain the clamping force of the stack at an appropriate level to achieve good stack performance.
Conventionally, the fuel cell stack is assembled using bolts and nuts or bands. For example, as shown in FIG. 8, a bolt 100 having a length greater than that of the stack is used and nuts 102 are tightened to both ends of the bolt 100 and, as shown in FIG. 9, a band 104 is fastened to the end plates being pressed by a press and then fixed by bolts.
However, in the method of assembling the fuel cell stack using bolts there is a large amount of dead volume, such as the space between the fuel cell stack and the long bolts, the nut portions extending to the outside of the end plates, etc., which is inappropriate for application to the vehicle.
Further, in the method of assembling the fuel cell stack using bands, in the event of a deviation in elastic properties or dimensions of each component of the fuel cell stack, the length of the assembled fuel cell stack increases, and thus it is necessary to prepare bands having various dimensions, which cannot appropriately cope with the fuel cell stacks having a large deviation in length. When the fuel cell stack is excessively fastened, the performance of the fuel cell stack is suitably reduced by the deformation of the gas diffusion layer, and accordingly this method cannot handle the change in the clamping force of the stack.
Alternatively, as shown in FIG. 10, a disc spring 106 is used as a surface pressure maintaining means between the end plate and the separator or, as shown in FIG. 11, a spring module 108 including a plurality of springs is used. However, in these methods, a separate pressure plate is provided, which increases the weight and volume, and the pressure plate is integrated with a current collector, which complicates an electricity leading structure through the current collector.
Accordingly, when such high stiffness metal elastic members such as a plate spring, a disc spring, a coil spring, etc. are used to provide a uniform surface pressure on the fuel cell stack, it is necessary to use an end plate including a high strength member or a reinforcing structure which can suitably endure a high local load of the metal elastic member. Accordingly, the weight and volume of the fuel cell stack increases, which makes it difficult to suitably optimize the layout of the fuel cell stack in an engine room. Further, since it is necessary to disassemble and assemble the metal elastic member such as a spring module in a state where the metal elastic member is clamped at a clamping pressure of 3 to 4 tons, it requires pressing equipment, which may be disadvantageous in terms of assembly and maintenance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.